Various methods in the prior art are utilized to measure the index of refraction of glass. One method entails utilizing an Abbe refractometer to measure the critical angle of total reflection. In accordance with this method, the index of refraction is derived from the measured value of the critical angle. This method is illustrated in a book entitled "Optics" by Eugene Hecht and Alfred Zajac, published by Addison-Wesley Publishing Company, Inc. February, 1979 (Copyright 1974), pp. 81-84.
Another method utilized to measure the index of refraction of glass entails measuring the reflectivity of glass, which reflectivity depends on the index of refraction. The relation between index of refraction and reflectivity is described, for example, by the Fresnel formula shown on p. 75 of the above-identified reference.
Yet another method utilized to measure the index of refraction of glass entails measuring the deflection angle of a light beam passing through a sample block of glass. This method is illustrated on pp. 62-63 of the above-identified reference.
All of the above methods suffer from drawbacks. For example, all of the above methods require the surface geometry of the glass whose index of refraction is to be measured to be known. For example, the radii of curvature of the surfaces must be measured to an accuracy of about 0.1% to obtain a measurement of index of refraction of comparable accuracy. In addition, the first two methods are measurements of reflection. This is a drawback when measuring the index of refraction of spectacles since spectacles are often antireflection coated.
In light of the above, there is a need for a method and apparatus for measuring the index of refraction of glass such as the glass of spectacle lenses without measuring the surface geometry of the glass.
In addition to the above, currently available lensmeters are limited since they only measure the effective power of a lens and they do not measure the curvature of the front and back surfaces, the thickness of the lens, or the index of refraction of the lens. For example, see: (a) an article entitled "The Dioptrometer: An Apparatus For Measuring The Power Of Commercial Lenses" by H. F. Kurtz, J. Opt. Soc. Am. and Rev. Sci. Instrum., Vol. 7, Nos. 1 to 12, 1923, pp. 103-125 and (b) U.S. Pat. No. 4,180,325, entitled "Lens Meter With Automated Readout," issued Dec. 25, 1979, W. E. Humphrey. As a result of this limitation, lens characteristics such as, for example, front vertex power, back vertex power, and the index of refraction of the lens (which determines the lens material) cannot be determined. This is a disadvantage in that it is often desirable to have these lens characteristics.
In light of the above, there is a need for a method and apparatus for measuring the following lens characteristics: (a) curvature of the front and back surfaces of the lens; (b) physical thickness; and (c) index of refraction of the lens to determine the front vertex power, back vertex power; and the lens material.